BA-09 · LESSON 04

UTXO set and chainstate

KEY DEFINITIONS

Terms used in this lesson

UTXO: Unspent transaction output: one discrete chunk of bitcoin that can be used as a transaction input.

Bitcoin Core separates block storage, indexes, mempool state, and the active UTXO database.

65 min Difficulty 2/5 Not started

VISUAL MODEL

See the whole mechanism before going step by step

Use the diagrams to orient yourself. Then the guided explanation below teaches every part in depth.

4 explanatory views

CONCEPT-SPECIFIC MAP · p2p propagationIndependent peers relay claims, then verify locallySelect any state to inspect its exact role.

RELATIONSHIP MAPIndependent peer relationships

01
ORIGINWallet or peer
A node receives a transaction, header, block, or request from an untrusted connection.
02
PARSEBounded message
Length, command, encoding, and resource limits are checked before deeper work.
03
VALIDATEUTXO set and chainstate
The UTXO set and chainstate stage applies local policy or consensus rules as appropriate.
04
STOREMempool or chainstate
Accepted data enters a local view that can differ temporarily from another node's view.
05
RELAYSelected peers
Relay spreads a valid candidate; it does not make the candidate final or universally known.

Rule shown: Network messages are invitations to verify, never authority to accept state.

STAGE 1 OF 5ORIGIN: Wallet or peer

A node receives a transaction, header, block, or request from an untrusted connection.

Governing relationship: Network messages are invitations to verify, never authority to accept state. Complete path: 1. ORIGIN, Wallet or peer. A node receives a transaction, header, block, or request from an untrusted connection. 2. PARSE, Bounded message. Length, command, encoding, and resource limits are checked before deeper work. 3. VALIDATE, UTXO set and chainstate. The UTXO set and chainstate stage applies local policy or consensus rules as appropriate. 4. STORE, Mempool or chainstate. Accepted data enters a local view that can differ temporarily from another node's view. 5. RELAY, Selected peers. Relay spreads a valid candidate; it does not make the candidate final or universally known.

1. State transitionWhat exists, what changes, and what results

BEFORE

Chainstate contains X:50,000 sats. Block B spends X and creates Y:30,000 and Z:19,000, with 1,000 sats fee.

MECHANISM

Validated or pending block bytes can be written to block files, while index records track hash, parent, work, status, and disk location.

AFTER

Block storage preserves history, chainstate represents spendability, and mempool represents tentative policy state.

Text equivalent: Starting from Chainstate contains X:50,000 sats. Block B spends X and creates Y:30,000 and Z:19,000, with 1,000 sats fee. The mechanism begins when Validated or pending block bytes can be written to block files, while index records track hash, parent, work, status, and disk location. The resulting lesson is Block storage preserves history, chainstate represents spendability, and mempool represents tentative policy state.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Receive and store block data

Validated or pending block bytes can be written to block files, while index records track hash, parent, work, status, and disk location.

Complete path: 1. Receive and store block data: Validated or pending block bytes can be written to block files, while index records track hash, parent, work, status, and disk location. 2. Connect chainstate: For an accepted block, the node consumes UTXO entries for inputs and creates entries for spendable outputs in an atomic database batch. 3. Maintain undo data: Undo records preserve information needed to restore spent outputs when disconnecting a block during a reorg. 4. Manage the mempool separately: Mempool entries depend on unconfirmed parents, conflicts, fees, and policy. A transaction can leave the mempool without changing confirmed chainstate. 5. Serve optional indexes: Transaction or block filters and other indexes are derived views. Disabling one can remove query capability without changing validation.

3. Rule and failure boundaryWhat accepts the result and where it can stop

ACCEPTED PATH

Each block transition consumes unique live inputs, conserves safe-integer value, creates unique outputs, records undo, and commits the whole batch atomically.

CHANGED CONDITION

Inflation or a second spend rejects and returns the exact initial UTXO set with no partial undo journal; disconnect removes created outputs and restores spent ones.

Important distinction: The mempool is part of Bitcoin's globally shared consensus state. Mempools differ across nodes because arrival order, configuration, capacity, and policy differ. Confirmed block and UTXO validation provide shared consensus behavior.

Text equivalent: The accepted path is enforced by Each block transition consumes unique live inputs, conserves safe-integer value, creates unique outputs, records undo, and commits the whole batch atomically. If the required condition changes, Inflation or a second spend rejects and returns the exact initial UTXO set with no partial undo journal; disconnect removes created outputs and restores spent ones.

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Learn the idea

Bitcoin Core stores related but distinct data. Block files contain serialized blocks. The block index stores metadata and chain relationships. Chainstate is the active UTXO database plus tip information. The mempool is an in-memory and optionally persisted set of policy-accepted unconfirmed transactions. Optional indexes support extra queries but are not consensus state.

GUIDED EXPLANATION1/5 ideas inspected

Receive and store block data

Validated or pending block bytes can be written to block files, while index records track hash, parent, work, status, and disk location.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“The mempool is part of Bitcoin's globally shared consensus state.”

Mempools differ across nodes because arrival order, configuration, capacity, and policy differ. Confirmed block and UTXO validation provide shared consensus behavior.

FINISH LEARNING FIRST

The questions unlock after every required learning activity

Inspect every guided idea, open the worked example, rebuild its mechanism, and complete the deterministic lesson tool. Your progress is saved automatically.